It’s all about shelf life!

Jan Eisby, CSO at surface treatment pioneer Vetaphone, discusses the effect of time on a substrate’s retained dyne level and explains the importance of knowing this during the whole production process.

The plastic films used in today’s package printing sector are non-absorbent and need to be surface treated to allow ink, lacquer, or adhesive to adhere securely.  This begins at extrusion with corona treatment to a certain dyne level.  But it is what happens to this dyne level before the substrate is used in production that is important.

For liquids to adhere securely to film, the surface energy must be matched.  Untreated film has very low surface energy so any liquid applied to it will stay on the surface in small beads.  Corona treating the material to increase its dyne level allows the liquid to adhere during the printing or converting process, producing packaging of high quality that is commercially saleable with reduced waste.

If you look at the stages of production involved, you can appreciate why corona treatment at extrusion is insufficient on its own.  Once the film is extruded, it is printed, then perhaps has an adhesive applied for lamination and maybe a lacquer coating before it becomes the end-product.

It is the time delay between each of these processes that causes the problem because all films suffer dyne decay from the moment they are extruded.  So, check the dyne level before you print and then before you apply adhesive, and again before coating.

I recommend testing with a dyne pen.  It’s a simple, reliable, and inexpensive method, especially if you compare it with the cost of attempting the next process with mis-matched surface energy levels between liquid and substrate.

The biggest time delay tends to occur between the film being extruded and when it is used for printing, so, storage of the film is critical.  The problem is that substrate manufacturers do not give clear guidance on this subject in the datasheets relating to the different films they produce.

There are four factors involved in dyne decay during storage: time, humidity, additives, and contamination.  These make up what we call the ‘decay profile’.  Some customers are better equipped to handle storage than others, and dyne decay is not necessarily consistent, so each case is different.

For example, we recently tested two types of PE.  One had 600ppm of additive, the other was a top-coated stock.  Both decayed initially at the same rate, but the top-coated material levelled out after one month at 42 dyne, while the other continued to decay over the three months of testing by which time it had decayed to 36 dyne.

So, the choice the customer had to make was spend more money upfront on top-coated material or have the cost and inconvenience of boost treating the cheaper stock after a period of storage.

Plasma is the best form of surface treatment if you need to obtain a high dyne level that can be retained over a long period.  Our Lab test shows that while corona treatment will achieve the desired dyne level for printing or adhesive application, the decay is relatively quick compared with ordinary plasma treatment, where the dyne level attained at treatment is higher and the decay is slower and flattens out at a higher dyne level.

Using high power plasma treatment achieves the highest dyne level that is retained for a longer period.  But it’s a question of cost – plasma is a far more complex and expensive process and corona is better suited in most cases.

Knowing your film parameters is essential.  How pure is it, how many ppm of additive does it have, what is its granulation?  These are all key elements, so you need to master the decay profile or go for ‘JIT’ production where storage is not an issue.

If you are storing film, keep the temperature in the 15 – 20 Celsius range, and the humidity between 50 – 60%, and make sure you keep the rolls wrapped to keep out dust and dirt.

Finally, to understand the converting parameters, you need ink/adhesive surface tension data from your suppliers.  If you then measure the film surface tension, you can use the curve profile to calculate the boost dosage needed to reach the correct dyne level at each process stage.

There are no short cuts, but Vetaphone can advise you at every step of the way to ensure you make best use of your time and money!